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| United States Patent |
5,112,680
|
|
Inaba
, et al.
|
May 12, 1992
|
Magnetic recording medium
Abstract
A magnetic recording medium comprising a nonmagnetic support having
provided thereon a magnetic layer comprising a ferromagnetic powder
dispersed in a binder, wherein said ferromagnetic powder has a specific
surface area of 40 m.sup.2 /g or more as measured by the BET method; said
magnetic layer further comprises Al.sub.2 O.sub.3 particles having an
average particle diameter of from about 0.1 to 0.5 .mu.m; and said binder
comprises the combination of (1) a vinyl chloride resin containing a
phosphate ester group represented by
##STR1##
wherein M.sub.1 and M.sub.2, which may be the same or different, each
represents Na, K, Li, H, N.sym.R.sub.3, or N.sym.HR.sub.2 wherein R
represents an alkyl group or a hydrogen atom, and (2) at least one resin
other than said vinyl chloride resin containing at least one of --SO.sub.3
M, --OSO.sub.2 M, --COOM, and
##STR2##
wherein M, M.sub.1, and M.sub.2, which may be the same or different, each
represents Na, K, Li, H, N.sym.R.sub.3, or N.sym.HR.sub.2 wherein R
represents an alkyl group or a hydrogen atom; provided that the weight
ratio of said resin other than said vinyl chloride resin to said vinyl
chloride resin is from about 0.4 to 2.5.
| Inventors:
|
Inaba; Hiroo (Kanagawa, JP);
Kakuta; Takeshi (Kanagawa, JP);
Yamauchi; Koichi (Kanagawa, JP);
Matsufuji; Akihiro (Kanagawa, JP)
|
| Assignee:
|
Fuji Photo Film Co., Ltd. (Kanagawa, JP)
|
| Appl. No.:
|
185214 |
| Filed:
|
April 25, 1988 |
Foreign Application Priority Data
| Apr 24, 1987[JP] | 100144/87 |
| Current U.S. Class: |
428/329; 428/408; 428/844.7; 428/900 |
| Intern'l Class: |
G11B 023/00 |
| Field of Search: |
428/694,329,900,332
|
References Cited
U.S. Patent Documents
| 4132827 | Jan., 1979 | Mukaida et al. | 428/329.
|
| 4439486 | Mar., 1984 | Yamada et al. | 428/332.
|
| 4594174 | Jun., 1986 | Nakayama et al. | 428/900.
|
| 4600521 | Jul., 1986 | Nakamura et al. | 428/694.
|
| 4613545 | Sep., 1986 | Chubachi et al. | 428/900.
|
| 4690863 | Sep., 1987 | Miyashi et al. | 428/900.
|
| 4707410 | Nov., 1987 | Hata et al. | 428/694.
|
| 4784907 | Nov., 1988 | Matsufuji et al. | 428/694.
|
| 4784914 | Nov., 1988 | Matsufuji et al. | 428/694.
|
| 4788103 | Nov., 1988 | Ohita et al. | 428/694.
|
| 4818606 | Apr., 1989 | Koyama et al. | 428/408.
|
| 4830923 | May., 1989 | Sumiya et al. | 428/694.
|
Primary Examiner: Thibodeau; Paul J.
Attorney, Agent or Firm: Sughrue, Mion, Zinn, Macpeak & Seas
Claims
What is claimed is:
1. A magnetic recording medium comprising a nonmagnetic support having
provided thereon a magnetic layer comprising a ferromagnetic powder
dispersed in a binder, wherein said ferromagnetic powder has a specific
surface area of 40 m.sup.2 /g or more as measured by the BET method; said
magnetic layer further comprising Al.sub.2 O.sub.3 particles having an
average particle diameter of from about 0.1 to 0.5 .mu.m; and said binder
comprising the combination of (1) a vinyl chloride resin containing a
phosphate ester group represented by
##STR6##
wherein M.sub.1 and M.sub.2, which may be the same or different, each
represents Na, K, Li, H, N.sym.R.sub.3, or N.sym.HR.sub.2 wherein R
represents an alkyl group or a hydrogen atom, and (2) at least one resin
other than said vinyl chloride resin containing at least one of --SO.sub.3
M, --OSO.sub.2 M, --COOM, and
##STR7##
wherein M, M.sub.1, and M.sub.2, which may be the same or different, each
represents Na, K, Li, H, N.sym.R.sub.3, or N.sym.HR.sub.2 wherein R
represents an alkyl group or a hydrogen atom; provided that the weight
ratio of said resin other than said vinyl chloride resin to said vinyl
chloride resin is from about 0.4 to 2.5.
2. A magnetic recording medium as claimed in claim 1, wherein the vinyl
chloride content of said vinyl chloride resin is from about 60 to 95 wt%.
3. A magnetic recording medium as claimed in claim 2, wherein the vinyl
chloride content of said vinyl chloride resin is from 80 to 95 wt%.
4. A magnetic recording medium as claimed in claim 1, wherein the content
of the phosphate group-containing monomer in said vinyl chloride resin is
from about 0.05 to 8% by weight.
5. A magnetic recording medium as claimed in claim 4, wherein the content
of the phosphate group-containing monomer in said vinyl chloride resin is
from 0.1 to 5% by weight.
6. A magnetic recording medium as claimed in claim 1, wherein said vinyl
chloride resin has a degree of polymerization of from about 150 to 600.
7. A magnetic recording medium as claimed in claim 6, wherein said vinyl
chloride resin has a degree of polymerization of from 200 to 450.
8. A magnetic recording medium as claimed in claim 1, wherein the weight
ratio of said resin other than said vinyl chloride resin to said vinyl
chloride resin is from about 0.4 to 1.5.
9. A magnetic recording medium as claimed in claim 8, wherein the weight
ratio of said resin other than said vinyl chloride resin to said vinyl
chloride resin is from 0.4 to 1.0.
10. A magnetic recording medium as claimed in claim 1, wherein said binder
further comprises an isocyanate.
11. A magnetic recording medium as claimed in claim 1, wherein said
magnetic layer comprises from about 1 to 19 parts by weight of said
Al.sub.2 O.sub.3 particles per 100 parts by weight of said ferromagnetic
powder.
12. A magnetic recording medium as claimed in claim 1, wherein said
ferromagnetic powder has a specific surface area of from 40 to 80 m.sup.2
/g as measured by the BET method.
13. A magnetic recording medium as claimed in claim 1, wherein the weight
ratio of the amount of said ferromagnetic powder to the total amount of
said magnetic layer is from 0.2 to 0.69.
14. A magnetic recording medium as claimed in claim 13, wherein the weight
ratio of the amount of said ferromagnetic powder to the total amount of
said magnetic layer is from 0.3 to 0.69.
15. A magnetic recording medium as claimed in claim 1, wherein said
ferromagnetic powder is Co-containing 8--Fe.sub.2 O.sub.3 powder having a
specific surface area S.sub.BET of about from 40 to 80 m.sup.2 /g as
measured by the BET method.
16. A magnetic recording medium as claimed in claim 15, wherein said
ferromagnetic powder is Co-containing 8--Fe.sub.2 O.sub.3 powder having a
specific surface area S.sub.BET of about from 45 to 80 m.sup.2 /g as
measured by the BET method.
17. A magnetic recording medium as claimed in claim 1, wherein said
magnetic layer further comprises carbon black having a primary particle
size of from 20 to 120 .mu.m in an amount of from 0.1 to 15 parts by
weight per 100 parts by weight of said ferromagnetic powder.
18. A magnetic recording medium as claimed in claim 17, wherein said carbon
black has a primary particle size of from 20 to 120 .mu.m in an amount of
from 0.5 to 12 parts by weight per 100 parts by weight of said
ferromagnetic powder.
Description
FIELD OF THE INVENTION
The present invention relates to a magnetic recording medium and, more
specifically, it relates to a magnetic recording medium which is excellent
in the reduction of the friction coefficient at the surface of the
magnetic layer and in the high frequency output, and in which
deterioration of S/N (signal/noise) ratio is prevented.
BACKGROUND OF THE INVENTION
It has been attempted to increase the packing density of magnetic powder by
introducing hydrophilic functional groups to a binder for use in a
magnetic recording medium. Particularly, in the case of a vinyl chloride
type resin, functional groups such as --OH and --COOH groups are
introduced for increasing the packing density of the dispersing matters.
However, if the vinyl chloride type resin alone is used as the binder, the
performance of the medium is remarkably degraded in view of the running
stability and the running durability because of its extremely poor
flexibility. In view of the above, resins excellent in flexibility such as
a polyurethane resin, a polyester resin, a phenoxy resin, an acrylonitrile
rubber, etc. are used as the second ingredient for providing the magnetic
recording medium with flexibility.
The above resins not containing functional groups have been used as the
second ingredient. However, since still higher density has been required
for magnetic recording media in recent years along with development in
short wavelength recording systems, acidic groups (such as carboxylic,
sulfonic, sulfate, and phosphate groups), amino groups, imino groups,
imido groups, amido groups, hydroxy groups, alkoxy groups, thiol groups,
halogen groups, silyl groups, etc. have now been included in the second
ingredient. Thus, the packing density of the magnetic powder can be
improved so as to be suitable for the short wavelength recording system.
However, if the dispersibility is increased excessively, the surface
smoothness of the magnetic recording medium is also increased, and the
drawback exists that the friction coefficient is undesirably increased.
Accordingly, it is difficult at present to improve the dispersibility on
one hand while suppressing the increase in the friction coefficient due to
the mirror-smooth surface on the other hand.
Further, it has been intended to decrease the particle size of
ferromagnetic powder used for the magnetic recording medium to permit
short wavelength recording in recent years. In this circumstances, a
problem such as reduction in the high frequency output and deterioration
of the S/N ratio have been the result which are the problems encountered
at present.
For reducing the friction coefficient, surface active lubricants, for
example, fatty acids, fatty acid amides, fatty acid alcohols, fatty acid
esters, or silicone oils, etc. have been used. Although the friction
coefficient can be reduced to some extent by such means, it is not yet
satisfactory for ensuring sufficient running performance and running
durability. Further, if they are added in an excess amount, disadvantages
such as head contamination occur due to exudation of the lubricants, and
this can not be considered an effective means.
In increasing the density of magnetic recording media in recent years,
their friction coefficient tends to be increased by making the magnetic
particles finer, increasing the packing density, and providing a
mirrorsmooth surface. However, both satisfactory running stability and
satisfactory electromagnetic conversion properties are difficult to attain
by the mere application of the surface active agents, carbon black, or
inorganic powders. Further, there has been a great problem in recent years
that the high frequency output and the S/N ratio of the magnetic medium
running in a deck are deteriorated.
In order to avoid these problems, method have been proposed in which poler
groups, such as a sulfonate group, a sulfate group, a carboxylate group, a
phosphate group, etc., are introduced into the binder, but they are still
insufficient in view of the electromagnetic conversion properties (as
described in U.S. Pat. Nos. 4,529,661, 4,521,486, and 4,613,545).
The present inventors have made earnest studies on such problems and, as a
result, have found that if the frictional resistance between the surface
of the magnetic recording medium and the glass portion at the periphery of
a magnetic head is high, destruction is caused at the surface of the glass
portion by the high speed sliding contact of the medium and, as result,
fine protrusions are formed on the glass surface and roughen the glass
surface. It has been found that the fine protrusions grow together with
the running time, which increase the spacing loss between the magnetic
surface of the medium and the magnetic head to result in the reduction in
the high frequency output and the S/N ratio. It has have also been found
that this phenomenon is greatly influenced particularly by the grain size
of the ferromagnetic powder and remarkably occurs in fine magnetic
particles having a specific surface area of 40 m.sup.2 /g or more as
measured by the BET method.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a magnetic recording
medium having satisfactory electromagnetic conversion characteristics and
excellent running stability, and running durability, by combining a resin
having hydrophilic functional groups with an alumina powder having an
appropriate grain size.
Other objects of the present invention will be apparent from the following
description.
The present inventors have studied the kind of functional groups of the
resin and the powder added to the magnetic recording medium for overcoming
the foregoing problems and, as a result, have found that both the increase
in the density of the magnetic recording medium and the surface
smoothness, and the reduction in the friction coefficient of the medium
surface can be attained.
It has now been found that the above objects of the present invention can
be attained by a magnetic recording medium comprising a nonmagnetic
support having provided thereon a magnetic layer comprising a
ferromagnetic powder dispersed in a binder, wherein the ferromagnetic
powder has a specific surface area S.sub.BET of 40 m.sup.2 /g or more as
measured by the BET method; the magnetic layer further comprises Al.sub.2
O.sub.3 particles having an average particle diameter of from about 0.1 to
0.5 .mu.m; and the binder comprises the combination of (1) (the first
ingredient) a vinyl chloride resin containing a phosphate ester group
represented by
##STR3##
wherein M.sub.1 and M.sub.2, which may be the same or different, each
represents Na, K, Li, H, N.sym.R.sub.3, or N.sym.HR.sub.2 wherein R
represents an alkyl group or a hydrogen atom, and (2) (the second
ingredient) at least one resin other than the vinyl chloride resin
containing at least one of --SO.sub.3 M, --OSO.sub.2 M, --COOM, and
##STR4##
wherein M, M.sub.1, and M.sub.2, which may be the same or different, each
represents Na, K, Li, H, N.sym.R.sub.3, or N.sym.HR.sub.2 wherein R
represents an alkyl group or a hydrogen atom; provided that the weight
ratio of the resin other than the vinyl chloride resin (the first
ingredient) to the vinyl chloride resin (the second ingredient) is from
about 0.4 to 2.5.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is now described in more detail below.
According to the present invention, the vinyl chloride resin containing the
phosphate ester group as the first ingredient is excellent in the
dispersibility not only of the magnetic powder but also, particularly, of
Al.sub.2 O.sub.3, and the magnetic properties of the medium are not
deteriorated if Al.sub.2 O.sub.3 is added. Further, when the vinyl
chloride resin containing the phosphate ester group is combined with the
functional group-containing resin as the second ingredient and used
together with Al.sub.2 OH.sub.3 having a suitable grain size, the
frictional resistance to the glass portion of the magnetic head can be
reduced remarkably and, as a result, deterioration in the high frequency
output and the S/N ratio after running as described above can be improved.
This is demonstrated as is shown by examples described later by the actual
measurement using a video tape and by measuring the friction coefficient
with a glass rod.
The vinyl chloride resin containing the phosphate ester group used as the
first ingredient in the present invention is prepared as a polymer or a
copolymer of phosphate ester group-containing monomers.
Vinyl chloride units provide, in cooperation with other constituent units,
an appropriate hardness and flexibility to the magnetic layer. Since the
abrasion resistance of the magnetic recording medium becomes poor if the
amount of vinyl chloride units is too small, whereas the solvent
solubility tends to be reduced if it is excessive, the vinyl chloride
content of the vinyl chloride resin is preferably from about 60 to 95 wt%,
and more preferably from 80 to 95wt%.
The vinyl monomer containing the phosphate ester group in the present
invention is a polymerizable vinyl monomer substituted with a phosphate
ester group, and examples thereof include acid phosphoxy ethylacrylate,
acid phosphoxy ethylmethacrylate, acid phosphoxy propylacrylate, acid
phosphoxy propylmethacrylate, 3-chloro-2-acid phosphoxy propylacrylate,
and 3-chloro-2-acid phosphoxy propylmethacrylate.
The phosphate ester group-containing vinyl monomer greatly contributes to
the improvement of the dispersibility of the magnetic powder. If the
amount of the phosphate group-containing monomer is too large, the
solubility of the copolymer to solvents is lowered failing to obtain a
uniform solution, the water resistance is deteriorated, and further, since
side reactions other than the cross-linking reaction occurs with
isocyanate compounds, the disadvantages exists that no cross linking
effect for the improvement of the abrasion resistance can be obtained, and
moreover the pot life of the magnetic coating composition is shortened. On
the other hand, if the amount of the phosphate group-containing monomer is
too small, the dispersibility of the magnetic powder tends to be
deteriorated. Accordingly, the content of the phosphate group-containing
monomer in the vinyl chloride resin is preferably from about 0.05 to 8%,
and more preferably from 0.1 to 5% by weight.
The vinyl chloride resin containing phosphate group can be obtained by the
known polymerization processes, for example, precipitation polymerization,
solution polymerization, suspension polymerization, emulsion
polymerization, etc. In the case of precipitation polymerization, methanol
or n-hexane which provides good solubility for the vinyl monomer
containing phosphate group, and in which the resultant polymer is
insoluble, is preferably used as a polymerization solvent, and methanol is
particularly preferred in view of the solvent solubility of the resulting
copolymer and from an economic point of view. The vinyl chloride resin can
be obtained as a fine powder in any of the solvents.
The polymerization degree of the vinyl chloride resin is preferably from
about 150 to 600, more preferably from 200 to 450, in view of the
mechanical strength and the adhesiveness of the magnetic layer and the
characteristics of the magnetic coating composition for the magnetic
layer. The resin having an average degree of polymerization of less than
150 is unsuitable since it provides only a weak coated surface when it is
applied to a support, whereas the resin having a degree of polymerization
of more than 600 increases the viscosity of the coating composition to
reduce the workability upon coating.
The functional group-containing resin other than vinyl chloride as the
second ingredient in the present invention can include, for example, an
acrylate/acrylonitrile copolymer, an acrylate/vinylidene chloride
copolymer, an acrylate/styrene copolymer, a methacrylate/acrylonitrile
copolymer, a methacrylate/ vinylidene chloride copolymer, a
methacrylate/styrene copolymer, a urethane elastomer, a nylon-silicone
resin, a nitrocellulose-polyamide resin, a polyvinyl fluoride resin, a
vinylidene chloride/acrylonitrile copolymer, a butadiene/acrylonitrile
copolymer, a polyamide resin, a polyvinyl butyral, cellulose derivatives
(cellulose acetate butyrate, cellulose diacetate, cellulose triacetate,
cellulose propionate, nitrocellulose, etc.), a styrene/butadiene
copolymer, a polyester resin, a chlorovinyl ether/acrylate copolymer, an
amino resin, thermoplastic resins of various kind of synthetic rubbers and
mixtures thereof. Among these, a urethane elastomer, a vinylidene
chloride/acrylonitrile copolymer, a butadiene/acrylonitrile copolymer,
cellulose derivatives, and a polyester resin are preferred, and a urethane
elastomer, a butadiene/acrylonitrile copolymer, and a polyester resin are
most preferred.
In the present invention, at least one of --SO.sub.3 M, --OSO.sub.2 M,
--COOM, and
##STR5##
wherein M, M.sub.1, and M.sub.2, which may be the same or different, each
represents Na, K, Li, H, N.sym.R.sub.3, or N.sym.HR.sub.2 wherein R
represents an alkyl group or a hydrogen atom is contained in the non-vinyl
chloride resin as the functional groups.
Acidic groups are preferred as the functional groups in view of the pot
life and the functional groups described above are particularly excellent
in view of the dispersibility.
In the present invention, the weight ratio of the second ingredient to the
first ingredient is preferably from about 0.4 to 2.5, more preferably from
0.4 to 1.5, and most preferably from 0.4 to 1.0. If the ratio is less than
about 0.4, the magnetic recording medium has an insufficient flexibility
resulting in disadvantages such as dropping of powder in view of the
running durability. On the other hand, if it exceeds about 2.5, the medium
becomes too soft and causes running troubles such as adhesion.
The binder in the present invention may further contain other polymerizable
monomers such as ethylene, propylene, vinyl acetate, etc. as a comonomer
of the vinyl chloride resin, and the homopolymers or copolymers of these
copolymerizable monomers may be used together with the first and second
ingredients. In the case of these copolymerizable monomer is a comonomer
of the vinyl chloride resin, the amount thereof is preferably from 1 to 20
wt% based on the amount of the vinyl chloride resin. In the case where the
polymer or copolymer of these copolymerizable monomer is used, the amount
of such polymer or copolymer is preferably from 0.5 to 25 wt% based on the
total amount of the first and second ingredients. Particularly, use of
ethylene as a comonomer unit for the vinyl chloride polymer is preferred
in view of the improvement in the solvent solubility.
In the present invention, a polyisocyanate may preferably be used together
with the first and second ingredients. The added amount thereof is
preferably from 0.2 to 56. 25 wt%, and more preferably from 0.25 to 56.25
wt% based on the total amount of the first and the second ingredients.
The polyisocyanate usable in the present invention can include isocyanates,
for example, tolylene diisocyanate, 4,4'-diphenylmethane diisocyanate,
hexamethylene diisocyanate, xylylene diisoicyanate,
naphthylene-1,5-diisocyanate, o-tuluidine diisocyanate, isophorone
diisocyanate, and triphenylmethane triisocyanate; reaction products of
these isocyanates and polyalcohols; and polyisocyanates formed by the
condensation of isocyanates. Among these compounds, tolylene diisocyanate,
4,4'-diphenylmethane diisocyanate, hexamethylene diisocyanate, and
isophorone diisocyanate are preferably used in the present invention.
These polyisocyanates are commercially available under the trade names of,
for example, Collonate L, Collonate HL, Collonate 2,030, Collonate 2,031,
Millionate MR, Millionate MTL (manufactured by Nippon Polyurethan Co.),
Takenate D-102, Takenate D-110N, Takenate D-200, Takenate D-202
(manufactured by Takeda Yakuhin Co.), Desmodur L, Desmodur IL, Dismodur N,
Dismodur HL (manufactured by Sumitomo Bayer Co.), etc., which may be used
alone or in a combination of two or more of them utilizing the difference
in curing reactivity. Further, a compound having a hydroxy group or an
amino group may be used together to promote the curing reaction. It is
desirable that these compounds are polyfunctional.
The ferromagnetic fine powder usable in the present invention can include
known ferromagnetic fine powders such as .gamma.--Fe.sub.2 O.sub.3,
Co-containing .gamma.--Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, Co-containing
Fe.sub.3 O.sub.4, .gamma.--FeO.sub.x, Co-containing .gamma.--FeO.sub.x
wherein 1.33.ltoreq..times.1.50, CrO.sub.2, Co-Ni-P alloy, Co-Ni-Fe-B
alloy, Fe-Ni Zn alloy, Ni-Co alloy, Co-Ni-Fe alloy, etc. Specifically,
they are described, for example, in Japanese Patent Publication Nos.
14090/69, 18372/70, 22062/72, 22513/72, 28466/71, 38755/81, 4286/72,
12422/72, 17284/72, 18509/72, 18573/72, 10307/64, 29280/73, 39639/73,
29605/83, 44254/85, Japanese Patent Application (OPI) No. 126605/84, U.S.
Pat. Nos. 3,026,215, 3,031,341, 3,100,194, 3,242,005, and 3,389,014. In
addition, hexagonal tabular barium ferrite may also be used. The term
"OPI" used herein means a published unexamined Japanese patent
application.
It is a further feature of the present invention that the ferromagnetic
powder has a specific surface area S.sub.BET of about 40 m.sup.2 /g or
more, preferably from 40 to 80 m.sup.2 /g, as measured by the BET method.
The ratio of the amount of the ferromagnetic powder to the total amount of
the magnetic layer at the dry state is preferably from 0.2 to 0.69 by
weight, more preferably from 0.3 to 0.69 by weight.
Dispersants, lubricants, antistatic agent, etc. may be dissolved in the
solvent and adsorbed on the surface of the ferromagnetic fine powder prior
to the dispersion for the respective purposes described later.
In the present invention, the magnetic layer contains Al.sub.2 O.sub.3
powder having an average grain size (particle diameter) from about 0.1 to
0.5 .mu.m as an abrasive agent. The conventional magnetic recording medium
generally contains a combination of 1 to 4 kinds of abrasive agents having
Moh's hardness of 6 or more such as .alpha.-alumina, molten alumina,
silicon carbide, chromium oxide, cerium oxide, corundum, artificial
diamond, .alpha.-iron oxide, garnet, emery (mainly composed of corundum
and magnetite), silica rock, silicon nitride, boron nitride, molybdenum
carbide, boron carbide, tungsten carbide, titanium carbide, tripoli,
diatomaceous earth, dolomite, etc. Among these inorganic powders, when
Al.sub.2 O.sub.3 powder having an average grain size of from about 0.1 to
0.5 .mu.m is used in combination with the binder as described above, the
advantage of remarkably reducing the friction coefficient and smoothing
the sliding movement with the glass portion is obtained. If the average
grain size is less than about 0.1 .mu.m, the effect of reducing the
friction coefficient is insufficient and, on the other hand, satisfactory
electromagnetic conversion properties can not be maintained if it is more
than about 0.5 .mu.m. The Al.sub.2 O.sub.3 powder is preferably added to
the magnetic recording medium in an amount of from about 1 to 19 parts per
100 parts by weight of the ferromagnetic powder. The effect of the present
invention can be attained if it is used in combination with other abrasive
agents than Al.sub.2 O.sub.3.
In the present invention, additives such as carbon black, a lubricant, a
dispersant, an antistatic agent, etc. can further be incorporated into the
magnetic layer.
In the present invention, carbon black can be incorporated into the
magnetic layer and furnace carbon black for rubber, thermal carbon black
for rubber, color carbon black, or acetylene carbon black can be used as
the carbon black. Specific example of abbreviation of the carbon blacks in
the U.S. include SAF, ISAF, IISAF, T, HAF, SPF, FF, FEF, HMF, GPF, APF,
SRF, MPF, ECF, SCF, CF, FT, MT, HCC, HCF, MCF, LFF, RCF, etc. and those
classified in ASTM Standard D-1765-82a can be used. The carbon black used
in the present invention preferably has an average grain size of from
about 5 to 1,000 m.mu. (as determined by an electron microscope), a
specific surface area of from about 1 to 800 m.sup.2 /g determined by the
nitrogen adsorption method, a pH value of from about 4 to 11 (according to
JIS K-6221-1982), and a dibutylphthalate (DBP) oil adsorption amount of
from 10 to 800 ml/100 g (according to JIS K-6221-1982). The grain size of
the carbon black used in the present invention is preferably from 5 to 100
m.mu. for reducing the surface electric resistance of the coated film, and
from 50 to 1,000 m.mu. for controlling the strength of the coated film.
Further, the carbon black of fine particles (100 m.mu. or less) can be
used for smoothing to reduce spacing loss, and carbon black of coarse
grains (50 m.mu. or more) is used with an aim of roughening the surface to
reduce the friction coefficient. Thus, the kind and the addition amount of
the carbon black used are dependent on the purpose required for the
magnetic recording medium.
Further, the carbon black may be used after the surface treating with the
dispersant described later, or being grafted with resins. Further, these
carbon blacks partially graphitized at the surface thereof by treatment at
a temperature of 200.degree. C. or more in a furnace upon producing carbon
black may also be used. Furthermore, hollow carbon black may specially be
used. It is preferred that the carbon black is used in an amount from
about 0.1 to 20 parts by weight per 100 parts by weight of the fine
ferromagnetic powder in the magnetic layer. Carbon blacks which can be
used in the present invention are described in Carbon Black Binran (Manual
for Carbon Black), edited by Carbon Black Association (1972).
The lubricant usable for the magnetic layer in the present invention can
include, for example, silicone oil, graphite, molybdenum disulfide, boron
nitride, graphite fluoride, fluorinated alcohol, polyolefin (e.g.,
polyethylene wax, etc.), polyglycol (e.g., polyethylene oxide wax, etc.),
alkyl phosphate ester, polyphenyl ether, tungsten disulfide, a fatty acid
ester formed of a monohydric fatty acid ester having from 10 to 20 carbon
atoms and one or more of mono-valent, di-valent, tri-valent, tetra-valent
and hexa-valent alcohols having from 3 to 12 carbon atoms, and fatty acid
esters formed of mono-basic fatty acids having from 10 or more carbon
atoms and monovalent to hexa-valent alcohols having a number of carbon
atoms providing the total number of carbon atoms of from 11 to 28. Fatty
acids, fatty acid amides, and fatty acid alcohols each having from 8 to 22
carbon atoms may also be used. Specific examples of the organic compound
lubricants include butyl caprylate, octyl caprylate, ethyl laurate, butyl
laurate, octyl laurate, ethyl myristate, butyl myristate, octyl myristate,
ethyl palmitate, butyl palmitate, octyl palmitate, ethyl stearate, butyl
stearate, octyl stearate, amyl stearate, anhydrosorbitan monostearate,
anhydrosorbitan distearate, anhydrosorbitan tristearate, anhydrosorbitan
tetrastearate, oleyl oleate, oleyl alcohol, lauryl alcohol, etc.
As the lubricant for use in the present invention, conventional lubricant
oil additives may also be used, and examples thereof include anti-oxidants
(alkyl phenole, etc.), rust inhibitors (naphthenic acid, alkenyl succinic
acid, dilauryl phosphate, etc.), oilly agents (rapeseed oil, lauryl
alcohol, etc.), extreme pressure agents (dibenzyl sulfide, tricresyl
phosphate, tributyl phosphite, etc.), cleaning dispersants, viscosity
index improvers, flow point depressants, defoaming agents, etc. These
lubricants are preferably added in an amount of from about 0.05 to 20
parts by weight per 100 parts by weight of the ferromagnetic powder. These
agents are described e.g., in Japanese Patent Publication Nos. 238898/68,
4041/73, 18482/73, 18221/69, 28043/72, and 56132/82, U.S. Pat. Nos.
3,423,233, 3,470,021, 3,492,235, 3,497,411, 3,523,086, 3,625,760,
3,630,772, 3,634,253, 3,642,539, 3,687,725 and 4,135,031, and IBM
Technical Disclosure Bulletin vol. 9, No. 7, p 779 (December 1966).
The dispersant usable in the present invention can include fatty acids
having from 10 to 22 carbon atoms such as caprylic acid, capric acid,
lauric acid, myristic acid, palmitic acid, stearic acid, oleic acid,
elaidic acid, linoleic acid, linolenic acid, stearolic acid (R.sub.1 COOH
in which R.sub.1 represents an alkyl group having from 9 to 21 carbon
atoms), alkali metal soaps thereof (Li, Na, K, etc.) alkaline earth metal
soaps thereof (Mg, Ca, Ba, etc.), Cu, Pb, etc. soaps of the fatty acids
described above; lecithin, etc. In addition, higher alcohols having 4 or
more carbon atoms, such as butanol, octanol, myristyl alcohol, stearyl
alcohol, as well as sulfate esters and phosphate esters thereof may also
be used. The dispersant is preferably added in an amount of from about
0.005 to 20 parts by weight per 100 parts by weight of the binder. The
dispersant may be previously deposited on the surface of the fine
ferromagnetic powder or the fine non-magnetic powder, or added during
dispersing the coating composition. These agents are described, for
example, in Japanese Patent Publication Nos. 28639/64, 17945/69, 18221/69,
39402/74, 15001/73, and U.S. Pat. Nos. 3,387,993 and 3,470,021.
The antistatic agent usable in the present invention can include, for
example, an electroconductive powders such as graphite, carbon black,
carbon black graft polymer; natural surface active agents such as saponin;
nonionic surface active agents such as alkylene oxides, glycerins,
glycidols, polybasic alcohols, polybasic alcohol esters, alkylphenol
ethylene oxide addition products; cationic surface active agents such as
higher alkyl amines, cyclic amines, hydantoin derivatives, amide amines,
ester amides, quarternary ammonium salts, pyridine and other heterocyclic
compounds, and phosphoniums or sulfoniums; anionic surface active agents
containing acidic groups such as carboxylic, sulfonic phosphoric, sulfate,
and phosphorate groups; amino acids; amphoteric surface active agents such
as amino sulfonic acids, sulfuric acid or phosphoric acid esters of amino
alcohols, and alkyl betaines. Examples of the surface active agent
compound usable as the antistatic agents are disclosed, for example, in
U.S. Pat. Nos. 2,271,623, 2,240,472, 2,288,226, 2,676,122, 2,676,924,
2,676,975, 2,691,566, 2,727,860, 2,730,498, 2,742,379, 2,739,891,
3,068,101, 3,158,484, 3,201,253, 3,210,191, 3,294,540, 3,415,649,
3,441,413, 3,442,654, 3,475,174, 3,545,974, West German Patent Application
(OLS) No. 1,942,665, British Patents 1,077,317 and 1,198,450, as well as
Ryohei Oda, Kaimennkasseizai no Gosei to sono Oyo (Synthesis and
Application of Surface Active Agent), (Maki Shoten 1972); A. W. Baily
Surface Active Agents (Inter Science Publication Corporated 1985);
Encyclopedia of Surface Active Agents, vol. 2, (Chemical Publishing
Company 1964); Kaimenkasseizai Binran (Surface Active Agent Manual), (6th
edition, Sagyo Tosho Co., Dec. 20, 1966); and Hideo Marushige, Taiden
Boshizai (Antistatic Agent) (Miyuki Shobo 1968), etc.
These surface active agents may be added alone or in combination. The
amount of the surface active agent used in the magnetic recording medium
of the present invention is preferably from about 0.01 to 10 parts by
weight per 100 parts by weight of the fine ferromagnetic powder. These
agents are used as an antistatic agent, but may often be used for other
purposes, for example, improvement in dispersibility, magnetic properties,
and lubricating properties, and as coating assistants. These agents are
described, for example, in Elektronik, Vol. 9 No. 7, page 779 (1961); ibid
No. 12, page 380 (1961); Kagaku Binran (Manual of Chemistry), application
section, pages 954 to 967, (Maruzen Co. 1980).
The organic solvent used in the present invention, used for dispersing,
kneading, and coating, can include ketones such as acetone, methyl ethyl
ketone, methyl isobutyl ketone, cyclohexanone, and isophorone; alcohols
such as methanol, ethanol, propanol, butanol, isobutanol, isopropanol, and
methyl cyclohexanol; esters such as methyl acetate, ethyl acetate, butyl
acetate, isobutyl acetate, isopropyl acetate, and ethyl lactate glycol
monoethylether acetate; ethers such as diethyl ether, tetrahydrofuran,
glycol dimethyl ether, glycol monoethyl ether and dioxane; tar type
aromatic hydrocarbons such as benzene, toluene, xylene, cresol,
chlorobenzene, and styrene; chlorinated hydrocarbons such as methylene
chloride, ethylene chloride, carbon tetrachloride, chloroform, ethylene
chlorohydrin, and dichlorobenzene; N,N-dimethyl formaldehyde; and hexane.
There is no particular restriction in the present invention for the method
of kneading and dispersing, and the order of adding each of the
ingredients is not limited. The magnetic coating composition can be
prepared by using a conventional kneader, for example, a 2-roll mill, a
3-roll mill, a ball mill, a pebble mill, a tron mill sand grinder, a
Szegvari attritor, a high speed impeller, a disperser, a high speed stone
mill, a high speed impact mill, a disper, a kneader, a high speed mixer, a
ribbon blender, a kneader, an intensive mixer, a tumbler, a blender, a
disperser, a homogenizer, a single-axis screw extruder, a double-axis
screw extruder, and a ultrasonic wave disperser. Details of the techniques
of kneading and dispersing are described, for example, in T. C. Patton
Paint Flow and Pigment Dispersion (John Wiley & Sons 1964), Kogyo Zairyo
(Industrial Material) Vol. 25, 37 (1977) by Shinichi Tanaka, and
publications cited therein. They are further described in U.S. Pat. Nos.
2,581,414 and 2,855,156. In the present invention, the magnetic coating
composition can be prepared by kneading and dispersing in accordance with
the methods as described in these publications.
The magnetic recording layer may be formed by optionally selecting the
ingredients as described above, dissolving and dispersing them in an
organic solvent, and coating as a coating composition on a support
followed by drying. The dry thickness of the magnetic layer is preferably
from about 1.5 to 7.0 .mu.m, and more preferably from 3.0 to 6.0 .mu.m. In
the case of preparing a magnetic tape, it is preferred that the thickness
of the support is from about 2.5 to 100 .mu.m, and preferably from about 3
to 70 .mu.m. In the case of a disk or a card-like material, the thickness
of the support is preferably from about 0.5 to 10 mm, and in the case of a
drum, it may used in a cylindrical shape. As the material for the support,
there can be used polyesters such as polyethylene terephthalate,
polyethylene naphthalate; polyolefins such as polypropylene, cellulose
derivatives such as cellulose triacetate and cellulose diacetate; vinyl
resins such as polyvinyl chloride; other plastics such as polycarbonate,
polyamide, polysulfone, etc. as well as metals such as aluminum and
copper, and ceramics such as glass. These supports may be subjected to
corona discharging treatment, plasma treatment, primer coating, heat
treatment, dustremoving treatment, metal vapor deposition, and alkali
treatment prior to the coating. These supports are described, for example,
in West German Patent No., 3338854A, Japanese Patent Application (OPI) No.
116926/84, U.S. Pat. No. 4,388,368; and Sen'i to Kogyo (Fibers and
Industry), vol. 31, pages 50 to 55, by Yukio Mitsuishi (1975).
As the method of coating the coating composition the magnetic recording
layer onto the support, there can be used air doctor coating, blade
coating, air knife coating, squeeze coating, dip coating, reverse roll
coating, transfer roll coating, gravure coating, kiss coating, cast
coating, and spray coating, as well as other methods. These methods are
specifically described e.g., in Coating Kogaku (Coating Technology), pages
253 to 277 (Asakura Shoten, Mar. 20, 1971).
After the magnetic layer is coated on a support by such a method, the
magnetic powder in the layer is magnetically oriented by a conventional
method as required while drying, and then the formed magnetic layer is
dried. The conveying speed for the support in this case is usually from
about 10 to 900 m/min and the drying temperature is usually controlled to
from about 20.degree. to 30.degree. C.
The magnetic recording medium according to the present invention is
prepared by further applying surface smoothing techniques or cutting into
a desired shape as required. These processes are described, for example,
in Japanese Patent Publication 23635/65 and 28368/64, and U.S. Pat. No.
3,473,960, Further, the methods described in Japanese Patent Publication
13181/67 which is a fundamental and important techniques in the relevant
field of art can also be applied to the present invention.
The fine ferromagnetic powder, the non-magnetic powder, the binders, the
additives, the solvents, and the supports, (optionally including a primer
layer, a backing layer, a backing primer layer) and the production
processes for the magnetic recording medium described in Japanese Patent
Publication No. 26890/81 can be applied to the present invention.
The present invention will now be described in more detail with reference
to specific example, but the present invention is not to be construed as
being limited thereto. Unless otherwise indicated, all parts, percents,
and ratios are by weight.
EXAMPLES 1 AND 2 AND COMPARATIVE EXAMPLES 1 TO 8
Each of the example composition was prepared as follows.
After charging a portion of the composition described below into a ball
mill followed by sufficiently kneading, the remaining portion of the
composition was charged in the ball mill and sufficiently kneaded. Then,
15 parts of Desmodur L75 (trade name of polyisocyanate compound,
manufactured by Bayer Co.) were added thereto and uniformly mixed and
dispersed to prepare a magnetic coating composition.
______________________________________
Co-containing .gamma.-Fe.sub.2 O.sub.3 powder
100 parts
Vinyl chloride resin (fist ingredient)
10 parts
Non-vinyl chloride resin (second ingredient)
10 parts
Al.sub.2 O.sub.3 powder 5 parts
Carbon black 5 parts
Lecithin 1 part
Oleic acid 1 part
Octyl laurate 1.5 parts
Butyl acetate 200 parts
Methyl ethyl ketone 100 parts
______________________________________
In Example and Comparative Examples, the kind of Co-containing
.gamma.--Fe.sub.2 O.sub.3 powder, vinyl chloride resin, resin other than
vinyl chloride (non-vinyl chloride resin), and Al.sub.2 O.sub.3 powder in
the composition were varied as follows.
EXAMPLE 1
Co-containing .gamma.--Fe.sub.2 O.sub.3 powder
Specific service area (BET method): 45 m.sup.2 /g
Coersive force Hc: 880 Oe
Vinyl chloride-vinyl acetate copolymer containing phosphate ester groups
Functional group content: 6.5.times.10.sup.-5 eq/g
Number average molecular weight: 16,000
Polymerization degree: 380
Carboxyl-containing polyurethane (TiM-3005, manufactured by Sanyo Kasei
Co.)
Functional group content: 1.times.10.sup.-5 eq/g
Number average molecular weight: 22,000
Al.sub.2 O.sub.3 powder
Average grain size: 0.2 .mu.m
EXAMPLE 2
Co-containing .gamma.--Fe.sub.2 O.sub.3 powder
Specific service area (BET method): 45 m.mu./g
Coersive force Hc: 880 Oe
Vinyl chloride-vinyl acetate copolymer containing phosphate ester group
(same as in Example 1)
Sulfonic group-containing polyurethane (UR-8300, manufactured by Toyo
Boseki Co.)
Number average molecular weight: 25,000
Functional group content: 3.4.times.10.sup.-5 eq/g
Al.sub.2 O.sub.3 powder
Average grain size: 0.2 .mu.m
EXAMPLE 8
Co-containing .gamma.--Fe.sub.2 O.sub.3 powder
Specific service area (BET method): 45 m.sup.2 /g
Coersive force Hc: 880 Oe
Vinyl chloride-vinyl acetate copolymer containing phosphate ester group
(same as in Example 1)
Carboxyl group-containing polyurethane (TiM-3005, manufactured by Sanyo
Kasei Co.)
(same as in Example 1)
Cr.sub.2 O.sub.3 powder
Average grain size: 0.5 .mu.m
COMPARATIVE EXAMPLE 1
Co-containing .gamma.--Fe.sub.2 O.sub.3 powder
Specific service area (BET method): 45 m.sup.2 /g
Coersive force Hc: 880 Oe
Vinyl chloride-vinyl acetate copolymer containing carboxyl group
(400.times.110A, manufactured by Nippon Zeon Co.)
Functional group content: 1.times.10.sup.-3 eq/g
Number average molecular weight: 15,000
Polymerization degree: 420
Polyurethane (N-2304, manufactured by Nippon Polyurethane Co.)
Functional group content: none
Number average molecular weight: 34,000
Al.sub.2 O.sub.3 powder
Average grain size: 0.2 .mu.m
COMPARATIVE EXAMPLE 2
Co-containing .gamma.--Fe.sub.2 O.sub.3 powder
Specific service area (BET method): 35 m.sup.2 /g
Coersive force Hc: 880 Oe
Vinyl chloride-vinyl acetate copolymer containing carboxyl group
(400.times.110A, manufactured by Nippon Zeon Co.)
(same as in Comparative Example 1)
Polyurethane (N-2304, manufactured by Nippon Polyurethane Co.)
(same as in Comparative Example 1)
Al.sub.2 O.sub.3 powder
Average grain size: 0.2 .mu.m
COMPARATIVE EXAMPLE 3
Co-containing .gamma.--Fe.sub.2 O.sub.3 powder
Specific service area (BET method): 30 m.sup.2 /g
Coersive force Hc: 880 Oe
Vinyl chloride-vinyl acetate copolymer containing carboxyl group
(400.times.110A, manufactured by Nippon Zeon Co.)
(same as in Comparative Example 1)
Polyurethane (N-2304, manufactured by Nippon Polyurethane Co.)
(same as in Comparative Example 1)
Al.sub.2 O.sub.3 powder
Average grain size: 0.2 .mu.m
COMPARATIVE EXAMPLE 4
Co-containing .gamma.--Fe.sub.2 O.sub.3 powder
Specific service area (BET method): 45 m.sup.2 /g
Coersive force Hc: 880 Oe
Vinyl chloride-vinyl acetate copolymer containing phosphate ester group
(same as in Example 1)
Polyurethane (N-2304, manufactured by Nippon Polyurethane Co.)
(same as in Comparative Example 1)
Al.sub.2 O.sub.3 powder
Average grain size: 0.2 .mu.m
COMPARATIVE EXAMPLE 5
Co-containing --Fe.sub.2 O.sub.3 powder
Specific service area (BET method): 30 m.sup.2 /g
Coersive force Hc: 880 Oe
Vinyl chloride-vinyl acetate copolymer containing phosphate ester group
(same as in Example 1)
Carboxyl group-containing polyurethane (TiM-3005, manufactured by Sanyo
Kasei Co.)
(same as in Example 1)
Al.sub.2 O.sub.3 powder
Average grain size: 0.2 .mu.m
COMPARATIVE EXAMPLE 6
Co-containing .gamma.--Fe.sub.2 O.sub.3 powder
Specific service area (BET method): 45 m.mu./g
Coersive force Hc: 880 Oe
Vinyl chloride-vinyl acetate copolymer containing phosphate ester group
(same as in Example 1)
Carboxyl group-containing polyurethane
(TiM-3005, manufactured by Sanyo Kasei Co.)
(same as in Example 1)
Al.sub.2 O.sub.3 powder
Average grain size: 0.06 .mu.m
COMPARATIVE EXAMPLE 7
Co-containing .gamma.--Fe.sub.2 O.sub.3 powder
Specific service area (BET method): 45 m.sup.2 /g
Coersive force Hc: 880 Oe
Vinyl chloride-vinyl acetate copolymer
containing phosphate ester group
(same as in Example 1)
Carboxyl group-containing polyurethane (TiM-3005, manufactured by Sanyo
Kasei Co.)
(same as in Example 1)
Al.sub.2 O.sub.3 powder
Average grain size: 0.7 .mu.m
In Examples and Comparative Examples, the electro-magnetic conversion
characteristics on a VHS deck and the friction coefficients to glass and
stainless steel were measured by the following procedures.
Video S/N
The difference between the S/N ratio of the sample tape and that of "Super
AGT 120" manufactured by Fuji Photographic Film Co. as the standard tape
was determined by using a noise meter (925C) manufactured by Shibasoku.
The noise level was measured with a high pass filter at 10KHz and low pass
filter at 4MHz. The VTR used was "NV-8300" manufactured by Matsushita Co.
Video S/N degradation
The difference between the video S/N after one minute running on the above
deck and that after one hour running on the above deck was measured.
Friction coefficient to a stainless steel pole
The sample video tape and a stainless steel pole were brought into contact
under a tension of 50 g (T.sub.1), and the tension (T.sub.2) required for
running the tape at a speed of 3.3 cm/sec under this condition was
measured. The friction coefficient .mu. of the video tape which is shown
in Table 1 was determined by the following formula:
.mu.=1/.pi..multidot.1nT.sub.2 /T.sub.1
Friction coefficient to a glass pole
The friction coefficient to a glass pole was determined in the same manner
as the friction coefficient to the stainless steel pole as described
above.
The results of the measurement are shown in Table 1.
TABLE 1
__________________________________________________________________________
Friction coefficient
Video S/N ratio
to stainless
Friction coefficient
Sample No.
Video S/N ratio
degradation
steel pole (.mu.)
to glass pole (.mu.)
__________________________________________________________________________
Example 1
+3.5 (dB)
-0.4 (dB)
0.25 0.20
Example 2
+3.3 .+-.0 0.26 0.20
Comparative
+2.0 -0.8 0.33 0.32
Example 8
Comparative
+3.2 -2.2 0.45 0.40
Example 1
Comparative
+2.0 -0.7 0.35 0.29
Example 2
Comparative
+1.3 .+-.0 0.27 0.20
Example 3
Comparative
+2.6 -0.3 0.27 0.22
Example 4
Comparative
+2.4 .+-.0 0.26 0.21
Example 5
Comparative
+3.7 -2.5 0.42 0.45
Example 6
Comparative
+1.2 -0.3 0.22 0.20
Example 7
__________________________________________________________________________
EXAMPLE 4
A magnetic coating composition having the formulation shown below was
prepared and coated on a nonmagnetic support made of polyethylene
terephthalate having a 10 .mu.m thickness in the same manner as in Example
1 so that the thickness of the magnetic layer after drying was 3.0 .mu.m.
______________________________________
Ferromagnetic Fe--Ni alloy powder
100 parts
Ni content: about 5 wt %
specific surface area
determined by BET method: 45 m.sup.2 /g
Vinylchloride-vinylactate copolymler
10 parts
containing phosphate ester group
(same as in Example 1)
Sulfonic group-containing polyurethane
8 parts
(UR-8300, manufactured by Toyo Boseki)
(same as in Example 2)
Carbon black (Vulcan XC-72,
1 part
manufactured by Cabot Co.)
grain size: 32 m.mu.
Al.sub.2 O.sub.3 powder (Average grain size: 0.2 .mu.m)
8 parts
Oleic acid 0.5 parts
Myristic acid 1.5 parts
Octyl laurate 3 parts
Methyl ethyl ketone 200 parts
Polyisocyanate (Collonate L,
10 parts
manufactured by Nippon Polyurethan)
______________________________________
Magnetic field orientation was applied to the nonmagnetic support coated
with the magnetic coating composition when the coating composition was not
yet dried, and after drying and calendering to the surface, it was slit to
8 mm widths to prepare a 8 mm video tape.
COMPARATIVE EXAMPLE 9
A tape was prepared in the same manner as in Example 4 except for replacing
the vinylchloridevinyl acetate copolymer containing phosphoric acid ester
group with a vinylchloride-vinyl acetate copolymer containing carboxyl
group (VMCH, manufactured by Union Carbide Co., number average molecular
weight: 21,000, polymerization degree: 340, functional group (--COOH)
content : 9.5.times.10.sup.-5 eq/g).
The sample tapes obtained in Example 4 and Comparative Example 9 were
tested as follows.
Video S/N
The difference between the S/N ratio of the sample tape and that of "Super
AGP 6 90" manufactured by Fuji Photographic Film Co. as the standard tape
was determined by using a noise meter (925C) manufactured by Shibasoku.
The noise level was measured with a high pass filter at 10KHz and low pass
filter at 4MHz. The VTR used was "FUJIX Z 600AF" manufactured by Fuji
Photo Film Co., Ltd.
Video S/N degradation
The difference between the video S/N ratio of the initial first pass and
that after 10 times pass running was measured.
Friction coefficient to stainless steel and glass poles
The sample video tape and each of a stainless steel pole and a glass pole
were brought into contact under a tension of 20 g (T.sub.1), and the
tension (T.sub.2) required for running the tape at a speed of 1.4 cm/sec
under this condition was measured. The friction coefficient of the video
tape was determined by the Following formula:
.mu.=1/.pi..multidot.1nT.sub.2 /T.sub.1
The results of the evaluation for the 8 mm video tapes prepared in Example
4 and Comparative Example 9 are shown in Table 2.
TABLE 2
______________________________________
Friction
Friction
Video S/N coefficient
coefficient
Video S/N ratio to stainless
to glass
Sample No.
ratio degradation
Steel pole
pole
______________________________________
Example 4
+0.5 dB -0.6 dB 0.23 0.25
Comparative
-0.5 dB -3.5 dB 0.27 0.39
Example 9
______________________________________
Apparent from the results shown in Tables 1 and 2, it is possible according
to the invention to obtain a magnetic recording medium having satisfactory
electromagnetic conversion characteristics, with no deterioration in the
high frequency output and the S/N ratio, and excellent in running
stability.
While the invention has been described in detail and with reference to
specific examples thereof, it will be apparent to one skilled in the art
that various changes and modifications can be made therein without
departing from the spirit and scope thereof.
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